When I run a parallel reading performan testing on a md raid1 device with
two NVMe SSDs, I observe very bad throughput in supprise: by fio with 64KB
block size, 40 seq read I/O jobs, 128 iodepth, overall throughput is
only 2.7GB/s, this is around 50% of the idea performance number.
The perf reports locking contention happens at allow_barrier() and
wait_barrier() code,
- 41.41% fio [kernel.kallsyms] [k] _raw_spin_lock_irqsave
- _raw_spin_lock_irqsave
+ 89.92% allow_barrier
+ 9.34% __wake_up
- 37.30% fio [kernel.kallsyms] [k] _raw_spin_lock_irq
- _raw_spin_lock_irq
- 100.00% wait_barrier
The reason is, in these I/O barrier related functions,
- raise_barrier()
- lower_barrier()
- wait_barrier()
- allow_barrier()
They always hold conf->resync_lock firstly, even there are only regular
reading I/Os and no resync I/O at all. This is a huge performance penalty.
The solution is a lockless-like algorithm in I/O barrier code, and only
holding conf->resync_lock when it is really necessary.
The original idea is from Hannes Reinecke, and Neil Brown provides
comments to improve it. Now I write the patch based on new simpler raid1
I/O barrier code.
In the new simpler raid1 I/O barrier implementation, there are two
wait barrier functions,
- wait_barrier()
Which in turns calls _wait_barrier(), is used for regular write I/O.
If there is resync I/O happening on the same barrier bucket index, or
the whole array is frozen, task will wait untill no barrier on same
bucket index, or the whold array is unfreezed.
- wait_read_barrier()
Since regular read I/O won't interfere with resync I/O (read_balance()
will make sure only uptodate data will be read out), so it is
unnecessary to wait for barrier in regular read I/Os, they only have to
wait only when the whole array is frozen.
The operations on conf->nr_pending[idx], conf->nr_waiting[idx], conf->
barrier[idx] are very carefully designed in raise_barrier(),
lower_barrier(), _wait_barrier() and wait_read_barrier(), in order to
avoid unnecessary spin locks in these functions. Once conf->
nr_pengding[idx] is increased, a resync I/O with same barrier bucket index
has to wait in raise_barrier(). Then in _wait_barrier() or
wait_read_barrier() if no barrier raised in same barrier bucket index or
array is not frozen, the regular I/O doesn't need to hold conf->
resync_lock, it can just increase conf->nr_pending[idx], and return to its
caller. For heavy parallel reading I/Os, the lockless I/O barrier code
almostly gets rid of all spin lock cost.
This patch significantly improves raid1 reading peroformance. From my
testing, a raid1 device built by two NVMe SSD, runs fio with 64KB
blocksize, 40 seq read I/O jobs, 128 iodepth, overall throughput
increases from 2.7GB/s to 4.6GB/s (+70%).
Open question:
- I am not comfortable with freeze_array() and unfreeze_array(), for
writing I/Os if devices failed, wait_barrier() may have race with
freeze_array(), I am still looking for a solution now.
Signed-off-by: Coly Li <colyli@xxxxxxx>
Cc: Shaohua Li <shli@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Neil Brown <neilb@xxxxxxx>
Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
Cc: Guoqing Jiang <gqjiang@xxxxxxxx>
---
drivers/md/raid1.c | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------------------------
drivers/md/raid1.h | 12 +++++------
2 files changed, 75 insertions(+), 62 deletions(-)
Index: linux-raid1/drivers/md/raid1.c
===================================================================
--- linux-raid1.orig/drivers/md/raid1.c
+++ linux-raid1/drivers/md/raid1.c
@@ -222,8 +222,8 @@ static void reschedule_retry(struct r1bi
spin_lock_irqsave(&conf->device_lock, flags);
list_add(&r1_bio->retry_list, &conf->retry_list);
- conf->nr_queued[idx]++;
spin_unlock_irqrestore(&conf->device_lock, flags);
+ atomic_inc(&conf->nr_queued[idx]);
wake_up(&conf->wait_barrier);
md_wakeup_thread(mddev->thread);
@@ -811,11 +811,12 @@ static void raise_barrier(struct r1conf
spin_lock_irq(&conf->resync_lock);
/* Wait until no block IO is waiting */
- wait_event_lock_irq(conf->wait_barrier, !conf->nr_waiting[idx],
+ wait_event_lock_irq(conf->wait_barrier,
+ !atomic_read(&conf->nr_waiting[idx]),
conf->resync_lock);
/* block any new IO from starting */
- conf->barrier[idx]++;
+ atomic_inc(&conf->barrier[idx]);
/* For these conditions we must wait:
* A: while the array is in frozen state
@@ -826,23 +827,21 @@ static void raise_barrier(struct r1conf
* conrresponding to idx.
*/
wait_event_lock_irq(conf->wait_barrier,
- !conf->array_frozen && !conf->nr_pending[idx] &&
- conf->barrier[idx] < RESYNC_DEPTH,
+ !atomic_read(&conf->array_frozen) &&
+ !atomic_read(&conf->nr_pending[idx]) &&
+ atomic_read(&conf->barrier[idx]) < RESYNC_DEPTH,
conf->resync_lock);
- conf->nr_pending[idx]++;
+ atomic_inc(&conf->nr_pending[idx]);
spin_unlock_irq(&conf->resync_lock);
}
static void lower_barrier(struct r1conf *conf, sector_t sector_nr)
{
- unsigned long flags;
long idx = get_barrier_bucket_idx(sector_nr);
- BUG_ON(conf->barrier[idx] <= 0);
- spin_lock_irqsave(&conf->resync_lock, flags);
- conf->barrier[idx]--;
- conf->nr_pending[idx]--;
- spin_unlock_irqrestore(&conf->resync_lock, flags);
+ BUG_ON(atomic_read(&conf->barrier[idx]) <= 0);
+ atomic_dec(&conf->barrier[idx]);
+ atomic_dec(&conf->nr_pending[idx]);
wake_up(&conf->wait_barrier);
}
@@ -852,36 +851,55 @@ static void lower_barrier(struct r1conf
*/
static void _wait_barrier(struct r1conf *conf, long idx)
{
- spin_lock_irq(&conf->resync_lock);
- if (conf->array_frozen || conf->barrier[idx]) {
- conf->nr_waiting[idx]++;
- /* Wait for the barrier to drop. */
- wait_event_lock_irq(
- conf->wait_barrier,
- !conf->array_frozen && !conf->barrier[idx],
- conf->resync_lock);
- conf->nr_waiting[idx]--;
- }
+ /* We need to increase conf->nr_pending[idx] very early here,
+ * then raise_barrier() can be blocked when it waits for
+ * conf->nr_pending[idx] to be 0. Then we can avoid holding
+ * conf->resync_lock when there is no barrier raised in same
+ * barrier unit bucket.
+ */
+ atomic_inc(&conf->nr_pending[idx]);
+ if (!atomic_read(&conf->barrier[idx]) &&
+ !atomic_read(&conf->array_frozen))
+ return;
- conf->nr_pending[idx]++;
+ /* After holding conf->resync_lock, conf->nr_pending[idx]
+ * should be decreased before waiting for barrier to drop.
+ * Otherwise, we may encounter a race condition because
+ * raise_barrer() might be waiting for conf->nr_pending[idx]
+ * to be 0 at same time.
+ */
+ spin_lock_irq(&conf->resync_lock);
+ atomic_inc(&conf->nr_waiting[idx]);
+ atomic_dec(&conf->nr_pending[idx]);
+ /* Wait for the barrier in same barrier unit bucket to drop. */
+ wait_event_lock_irq(conf->wait_barrier,
+ !atomic_read(&conf->array_frozen) &&
+ !atomic_read(&conf->barrier[idx]),
+ conf->resync_lock);
+ atomic_inc(&conf->nr_pending[idx]);
+ atomic_dec(&conf->nr_waiting[idx]);
spin_unlock_irq(&conf->resync_lock);
}
+/* Very similar to _wait_barrier(), but only wait if array is frozen.
+ */
static void wait_read_barrier(struct r1conf *conf, sector_t sector_nr)
{
long idx = get_barrier_bucket_idx(sector_nr);
+ atomic_inc(&conf->nr_pending[idx]);
+ if (!atomic_read(&conf->array_frozen))
+ return;
+
spin_lock_irq(&conf->resync_lock);
- if (conf->array_frozen) {
- conf->nr_waiting[idx]++;
- /* Wait for array to unfreeze */
- wait_event_lock_irq(
- conf->wait_barrier,
- !conf->array_frozen,
- conf->resync_lock);
- conf->nr_waiting[idx]--;
- }
- conf->nr_pending[idx]++;
+ /* Wait for array to unfreeze */
+ atomic_inc(&conf->nr_waiting[idx]);
+ atomic_dec(&conf->nr_pending[idx]);
+ wait_event_lock_irq(conf->wait_barrier,
+ !atomic_read(&conf->array_frozen),
+ conf->resync_lock);
+ atomic_dec(&conf->nr_waiting[idx]);
+ atomic_inc(&conf->nr_pending[idx]);
spin_unlock_irq(&conf->resync_lock);
}
@@ -902,11 +920,7 @@ static void wait_all_barriers(struct r1c
static void _allow_barrier(struct r1conf *conf, long idx)
{
- unsigned long flags;
-
- spin_lock_irqsave(&conf->resync_lock, flags);
- conf->nr_pending[idx]--;
- spin_unlock_irqrestore(&conf->resync_lock, flags);
+ atomic_dec(&conf->nr_pending[idx]);
wake_up(&conf->wait_barrier);
}
@@ -933,7 +947,7 @@ static int get_all_pendings(struct r1con
int ret;
for (ret = 0, idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
- ret += conf->nr_pending[idx];
+ ret += atomic_read(&conf->nr_pending[idx]);
return ret;
}
@@ -944,7 +958,7 @@ static int get_all_queued(struct r1conf
int ret;
for (ret = 0, idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
- ret += conf->nr_queued[idx];
+ ret += atomic_read(&conf->nr_queued[idx]);
return ret;
}
@@ -965,7 +979,7 @@ static void freeze_array(struct r1conf *
* of conf->nr_pending[]) before we continue.
*/
spin_lock_irq(&conf->resync_lock);
- conf->array_frozen = 1;
+ atomic_set(&conf->array_frozen, 1);
wait_event_lock_irq_cmd(
conf->wait_barrier,
get_all_pendings(conf) == get_all_queued(conf)+extra,
@@ -977,7 +991,7 @@ static void unfreeze_array(struct r1conf
{
/* reverse the effect of the freeze */
spin_lock_irq(&conf->resync_lock);
- conf->array_frozen = 0;
+ atomic_set(&conf->array_frozen, 0);
wake_up(&conf->wait_barrier);
spin_unlock_irq(&conf->resync_lock);
}
@@ -2295,12 +2309,16 @@ static void handle_write_finished(struct
conf->mddev);
}
if (fail) {
+ /* set sector_nr before r1_bio add into conf->bio_end_io_list,
+ * we can't touch r1_bio once it is in this list, because
+ * it might be freed by raid_end_bio_io() in raid1d()
+ */
+ sector_nr = r1_bio->sector;
spin_lock_irq(&conf->device_lock);
list_add(&r1_bio->retry_list, &conf->bio_end_io_list);
- sector_nr = r1_bio->sector;
- idx = get_barrier_bucket_idx(sector_nr);
- conf->nr_queued[idx]++;
spin_unlock_irq(&conf->device_lock);
+ idx = get_barrier_bucket_idx(sector_nr);
+ atomic_inc(&conf->nr_queued[idx]);
md_wakeup_thread(conf->mddev->thread);
} else {
if (test_bit(R1BIO_WriteError, &r1_bio->state))
@@ -2410,7 +2428,6 @@ static void raid1d(struct md_thread *thr
struct r1conf *conf = mddev->private;
struct list_head *head = &conf->retry_list;
struct blk_plug plug;
- sector_t sector_nr;
long idx;
md_check_recovery(mddev);
@@ -2427,11 +2444,8 @@ static void raid1d(struct md_thread *thr
r1_bio = list_first_entry(&tmp, struct r1bio,
retry_list);
list_del(&r1_bio->retry_list);
- sector_nr = r1_bio->sector;
- idx = get_barrier_bucket_idx(sector_nr);
- spin_lock_irqsave(&conf->device_lock, flags);
- conf->nr_queued[idx]--;
- spin_unlock_irqrestore(&conf->device_lock, flags);
+ idx = get_barrier_bucket_idx(r1_bio->sector);
+ atomic_dec(&conf->nr_queued[idx]);
if (mddev->degraded)
set_bit(R1BIO_Degraded, &r1_bio->state);
if (test_bit(R1BIO_WriteError, &r1_bio->state))
@@ -2452,10 +2466,9 @@ static void raid1d(struct md_thread *thr
}
r1_bio = list_entry(head->prev, struct r1bio, retry_list);
list_del(head->prev);
- sector_nr = r1_bio->sector;
- idx = get_barrier_bucket_idx(sector_nr);
- conf->nr_queued[idx]--;
spin_unlock_irqrestore(&conf->device_lock, flags);
+ idx = get_barrier_bucket_idx(r1_bio->sector);
+ atomic_dec(&conf->nr_queued[idx]);
mddev = r1_bio->mddev;
conf = mddev->private;
@@ -2571,7 +2584,7 @@ static sector_t raid1_sync_request(struc
* If there is non-resync activity waiting for a turn, then let it
* though before starting on this new sync request.
*/
- if (conf->nr_waiting[idx])
+ if (atomic_read(&conf->nr_waiting[idx]))
schedule_timeout_uninterruptible(1);
/* we are incrementing sector_nr below. To be safe, we check against
@@ -3224,7 +3237,7 @@ static void *raid1_takeover(struct mddev
conf = setup_conf(mddev);
if (!IS_ERR(conf))
/* Array must appear to be quiesced */
- conf->array_frozen = 1;
+ atomic_set(&conf->array_frozen, 1);
return conf;
}
return ERR_PTR(-EINVAL);
Index: linux-raid1/drivers/md/raid1.h
===================================================================
--- linux-raid1.orig/drivers/md/raid1.h
+++ linux-raid1/drivers/md/raid1.h
@@ -6,7 +6,7 @@
*/
#define BARRIER_UNIT_SECTOR_BITS 17
#define BARRIER_UNIT_SECTOR_SIZE (1<<17)
-#define BARRIER_BUCKETS_NR (PAGE_SIZE/sizeof(long))
+#define BARRIER_BUCKETS_NR (PAGE_SIZE/sizeof(atomic_t))
/* will use bit shift later */
static inline long get_barrier_bucket_idx(sector_t sector)
@@ -74,11 +74,11 @@ struct r1conf {
*/
wait_queue_head_t wait_barrier;
spinlock_t resync_lock;
- int *nr_pending;
- int *nr_waiting;
- int *nr_queued;
- int *barrier;
- int array_frozen;
+ atomic_t *nr_pending;
+ atomic_t *nr_waiting;
+ atomic_t *nr_queued;
+ atomic_t *barrier;
+ atomic_t array_frozen;
/* Set to 1 if a full sync is needed, (fresh device added).
* Cleared when a sync completes.
--
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